1. Field of the Invention
The present invention relates to an in-focus detection apparatus suitable for a still camera or a video camera and, more particularly, to an in-focus detection apparatus in which an exit pupil of a photographing lens (objective lens) is divided into a plurality of areas, a plurality of object images are formed on a surface of an image pickup means using light beams passing through the divided areas, and an autocorrelation of image information obtained by the image pickup means is calculated, thereby detecting an in-focus state of the photographing lens.
2. Related Background Art
Conventionally, as a passive type focus detection system, which utilizes a light beam transmitted through an objective lens, and has relatively high in-focus detection precision, an in-focus detection apparatus utilizing a plurality of pieces of image information based on light beams passing through a plurality of different areas on a pupil of a photographing lens is known (U.S. Pat. No. 3,511,156, and the like). For example, U.S. Pat. No. 4,185,191 proposes an apparatus in which a movable portion of such an in-focus detection apparatus is omitted.
FIGS. 36 to 38 are enlarged views of an in-focus detection apparatus proposed in this patent, and its portion.
An optical system shown in FIG. 36 includes a photographing lens (objective lens) 601 and a detachable mirror 605. In a normal photographing mode, the mirror 605 is escaped outside a photographing optical path. At this time, an object image (image information) based on a light beam transmitted through the photographing lens 601 is formed on the surface of an image pickup means 602,
On the other hand, in an in-focus detection mode, as shown in FIG. 36, the mirror 605 is inserted from the position outside the optical axis into the optical path. Two object images based on light beams passing through two different areas (pupil areas) 603 and 604 on a pupil surface of the photographing lens 601 are reflected by the mirror 605, and are guided toward an in-focus information detection element 606.
The in-focus information detection element 606 has an arrangement shown in FIGS. 37 and 38. FIG. 37 shows a focusing state of light beams when the photographing lens 601 is set in an in-focus state, and FIG. 38 shows a focusing state of light beams when the lens 601 is set in a near-focus state (light-receiving elements are located behind the image formation plane).
In FIGS. 37 and 38, a lens member 621 consists of an array of a plurality of small lenses for separately imaging incident light beams, and a light-receiving means 622 consists of en array of pairs of light-receiving elements for receiving the light beams transmitted through the small lenses.
Of these light-receiving elements, light-receiving elements S.sub.a, T.sub.a, U.sub.a, V.sub.a, . . . receive a light beam 624 passing through the pupil area 604, and light-receiving elements S.sub.b, T.sub.b, U.sub.b, V.sub.b, . . . receive a light beam 623 passing through the pupil area 603.
In the in-focus state shown in FIG. 37, of a light beam from a certain portion of an object, a light beam passing through the pupil area 604 forms an image on the light-receiving element T.sub.a, and of the light beam from the same portion of the object, a light beam passing through the pupil area 603 forms an image on the light-receiving element T.sub.b.
Similarly, of light beams from other portions of the object, light beams passing through the pupil areas 603 and 604 respectively form images on the pairs of neighboring light-receiving elements S.sub.a and S.sub.b, U.sub.a and U.sub.b, and V.sub.a and V.sub.b.
In contrast to this, when the photographing lens is set in the near-focus state, as shown in FIG. 38, a light beam 623 corresponding to a light beam 624, which forms an image on the light-receiving element T.sub.a, forms an image on, e.g., the light-receiving element U.sub.b. Similarly, light beams from other portions form images on the light-receiving elements shifted one by one from the paired light-receiving elements.
Thus, when signals received by a light-receiving element group (a group a) consisting of the light-receiving elements S.sub.a, T.sub.a, U.sub.a, V.sub.a, . . . are compared with signals received by a light-receiving element group (a group b) consisting of the light-receiving elements S.sub.b, T.sub.b, U.sub.b, V.sub.b, . . . , the in-focus state of the photographing lens 601 is detected.
Furthermore, the defocusing amount and direction of the photographing lens 601 are detected by calculating a correlation of the output signals from the groups a and b.
In general, a conventional in-focus detection apparatus requires a certain relative positional precision between the image pickup element and the in-focus information detection element. In particular, when the area of the image pickup element is small, it is difficult to detect a position with high precision in a manufacturing process. When the positional relationship between the image pickup means 602 and the in-focus information detection element 606 is adversely affected by aging or a change in temperature, in-focus detection precision is impaired. Since extra detection means (lenses, light-receiving elements, and the like) for in-focus detection are required in addition to a photographing optical system for a photographing operation, the entire apparatus is complicated, and cost is increased. Since the in-focus information detection element 606 must be arranged outside the optical path of the photographing optical system, a compact structure of the camera is disturbed. Furthermore, in order to detect an in-focus position, calculations must be performed while shifting data from the two light-receiving element groups so as to find out a maximum correlation position, resulting in a long calculation time. Since a lens for splitting an optical path is used in addition to the photographing lens, a distance measurement system suffers from shading, distortion, and the like, and detection precision is impaired.